The present invention relates to an acrylic resin plastisol coating compound suitable for use as an undercoating material for protection of car bodies or as a sealing material for car bodies.
It has been common practice to apply an undercoating material to the wheel house, floor underside, and lower exterior of an automobile for their protection from chipping (paint film peeling by pebbles hitting against them). Such undercoating materials are mostly based on polyvinyl chloride resin (PVC) and polyvinyl chloride/polyvinyl acetate copolymer resin. In addition, a PVC-based sealing material is applied to panel joints of a car body for dust-and water-proofing.
Unfortunately, the conventional undercoating materials and sealing materials (which are composed mainly of polyvinyl chloride resin or polyvinyl chloride/polyvinyl acetate copolymer resin) are liable to give off, upon combustion, hydrogen chloride, dioxin, etc. detrimental to environments. So, chlorine-free substitutes have been sought for.
A promising substitute for coating compound based on polyvinyl chloride resin is one based on acrylic resin. However, no satisfactory product has been obtained yet which forms coating film with good properties and adhesion required of the undercoating material and sealing material for automotive use.
Improvement of acrylic resin in adhesion to the adherend (car body) was contemplated by incorporation with polyamide resin. However, the result was poor storage stability and insufficient adhesion of the coating compound.
Another way to improve adhesion is by incorporation with blocked isocyanate. This adhesion improver poses no problem with storage stability; however, conventional blocked isocyanate intended for PVC-based coating compounds does not provide sufficient adhesion. Moreover, the coating compound containing such blocked isocyanate has the disadvantage that the resulting coating film suffers interfacial failure between electrodeposited plate and coating film before it is broken by abrasion in the abrasion test such as nut-ropping test involving continuous weak shocks. Thus, it does not meet the requirements.
In general, any adhesion improver is more effective according as its molecular weight increases. However, any adhesion improver with a high molecular weight is viscous per se, and hence it deteriorates the workability of the coating compound containing it. Although it is possible to improve the workability by increasing the amount of plasticizer, the additional plasticizer deteriorates adhesion and characteristic properties.